The oxygen saturation measurement or also measurement of the oxygen content of blood is well known since the 1960ies as so-called pulse oxymetry. This method utilizes the fact that the color of blood depends on its oxygen saturation. For the transport of oxygen in blood, hemoglobin (Hb) is responsible, which by addition of O2 turns into oxyhemoglobin (O2Hb). When much oxygen is bound in the blood, the same has a red shade. In the case of a lower oxygen content in blood, the color of blood changes in the direction of a blue shade. This effect is due to the optical properties of the hemoglobin molecule. An oximeter measures the color differences of blood and therefrom calculates the oxygen saturation content.
The physical basis of this measurement method is the absorption of light according to the Lambert-Beer law. This law states that the total absorption of a medium which consists of different substances is the sum of the individual absorptions. The absorption of a substance depends on its concentration, the thickness and the material constant. The material constant is the molar extinction coefficient of the substance.
When radiation of a certain intensity is passed through a medium, the intensity decreases upon passage through the absorbing medium. The substance hemoglobin, which should be analysed for determining the oxygen saturation, chiefly consists of four constituents. These are O2Hb and Hb as functional fraction as well as COHb (carboxyhemoglobin) and MetHb (metahemoglobin) as dysfunctional fraction. The aforementioned constituents of hemoglobin have different absorption characteristics for light. These different absorption properties can be evaluated by means of a spectrophotometric measurement method and be used for determining the oxygen saturation. These connections are well known among experts (cf. also FIG. 1).
So-called pulse oximeters are known. These apparatuses used in the non-invasive measurement of the oxygen saturation utilize the variable strength of the absorber substance blood, i.e. the change of the distance covered by the light when passing through the absorber substance. The change in strength results from the pulse. Due to the heartbeat, a pulse wave runs through the arteries. The pulse wave generates a rhythmic expansion of the blood vessels and thus effects that for instance the finger—slightly—expands with each heartbeat. According to the Lambert-Beer law, the variable path length of the light passing through the absorber substance with a constant concentration of the blood oxygen causes a changing absorption. This change in absorption is used for determining the oxygen saturation.
One particularity of pulse oxymetry consists in that during the measurement a different absorber substance or different properties of the absorber substance must be considered each time. For instance, the fingers or the ear lobes (as examples of typical measurement sites of pulse oximeters) of different people are different. Therefore, the pulse oximeter, which e.g. in the form of a clip is put onto the finger or the ear lobe, must each be adapted to the new medium. The consequence is that there must be a very great dynamic range of the measuring device coupled with the pulse oximeter. In addition to the varying individual properties of each subject, the above-described regular fluctuations occur as a result of the propagating pulse wave in the body. In conventional pulse oxymetry, the fluctuations caused by the pulse wave are utilized to eliminate the individual, but constant physical properties of the persons to be measured from the measurement result. For this purpose, the rhythm of the heartbeat is extracted from the measurement signals, so that the measurements are in a defined relationship with the heartbeat. If these measured values, which are correlated in time with the pulse wave, then are subtracted, the constant part of absorption caused by the tissue then is eliminated due to the subtraction. The above explanations clearly illustrate that the pulse wave is an essential prerequisite for performing the blood oxygen measurement in connection with the pulse oxymetry.
The known pulse oximeters employ light emitting diodes (LEDs) with two wavelengths for determining hemoglobin and oxyhemoglobin. The other constituents of hemoglobin are considered only rarely. In a measurement with 2 LEDs, the other light-absorbing constituents of blood are included in the measurement result merely as errors. Typically, LEDs with the wavelengths of 660 nm and 940 nm are used. If the amounts of further hemoglobin constituents should also be determined, a further light emitting diode with a specific wavelength, which corresponds to the absorption behavior of the substance to be identified, must be used for each constituent. Correspondingly, there are pulse oximeters with e.g. four or five light emitting diodes.
A conventional pulse oximeter cannot be used when the blood does not pulsate, as is already indicated by the name “pulse oximeter”. The expansion of a vessel necessary for pulse oxymetry does not exist in the case of a measurement of the oxygen saturation outside the human body.
It is an object of the invention to provide an oximeter which supplies reliable measurement results when the blood does not pulsate, in particular when the blood is outside the living being to be examined.